Quenching of excitation energy

Recently, evidence has been presented that during steady state photosynthesis energy absorbed by PSII and not used for electron transport is dissipated via non-photochemical and non-radiative quenching of excitation energy [5] and that this high energy... 

In addition to the processes that can compete with fluorescence within the molecule itself, external actions can rob the molecule of excitation energy. Such an action or process is referred to as quenching. Quenching of fluorescence can occur because the dye system is too warm, which is a very common phenomenon. Solvents, particularly those that contain heavy atoms such as bromine or groups that ate detrimental to fluorescence in a dye molecule, eg, the nitro group, ate often capable of quenching fluorescence as ate nonfluorescent dye molecules. 

Herkstroeter and Hammond found support for this postulate from a flash photolysis study. They were able to measure directly the rate of sensitizer quenching (energy transfer) by cis- and fra/w-stilbene. When a sensitizer triplet had insufficient excitation energy to promote fims-stilbene to its triplet state, the energy deficiency could be supplied as an activation energy. The decrease in transfer rate as a function of excitation energy of the sensitizer is given by... 

Bimolecular reactions with paramagnetic species, heavy atoms, some molecules, compounds, or quantum dots refer to the first group (1). The second group (2) includes electron transfer reactions, exciplex and excimer formations, and proton transfer. To the last group (3), we ascribe the reactions, in which quenching of fluorescence occurs due to radiative and nonradiative transfer of excitation energy from the fluorescent donor to another particle - energy acceptor. 

Energy transfer entails the excitation of a molecule that during the lifetime of the excited state passes its excitation energy to another molecule. The loss of excitation energy from the initial excited species (the donor) results in quenching of the luminescence of the energy donor and may result in luminescence from the energy recipient (acceptor), which becomes excited in the process. 

Figure 7.16 Photoredox system for the oxidation of water to oxygen based on oxidative quenching of excited Ru(bpy)2+ by the sacrificial Co(NH3)5C12+ Reprinted from C. Kutal, Photochemical Conversion and Storage of Solar Energy , Journal of Chemical Education, Volume 60 (10), 1983. American Chemical Society...

The effects of photophysical intermolecular processes on fluorescence emission are described in Chapter 4, which starts with an overview of the de-excitation processes leading to fluorescence quenching of excited molecules. The main excited-state processes are then presented electron transfer, excimer formation or exciplex formation, proton transfer and energy transfer. 

The relatively long lifetimes of the excited states of these complexes have made them particularly attractive in the study of electron and energy-transfer quenching of excited states through organic bridges. In addition, the more positive redox potentials of these ions, compared with their pentaammine counterparts, mean that the mixed-valence ions are not air sensitive, thus facilitating spectroscopic measurements. 

Here we do not discuss the delocalization of the energy initially localized on one of the molecules owing to its nonradiative transfer to molecule-acceptors30 (see, e.g., Ref. 36). A detailed analysis of dissipation of excitation energy of a molecule-donor owing to concentration quenching by molecule-acceptors is presented in a recent review by Burstein.301... 

Fig. 9. Plots of log(k[M-1 s 1] for fluorescence quenching of excited states [21, 40]. The solid curve is a Rehm-WeBer plot and the broken one a Marcus plot, both with X = 9.6 kcal mol-1 = 40 kJ mol-1. The dotted curve corresponds to a Marcus plot with X = 38 kcal mol-1 = 159 kJ mol-1 (X = reorganization energy, AG° = corrected standard free energy change of electron transfer) — taken from Ref. [lb]...

With site-directed mutation and femtosecond-resolved fluorescence methods, we have used tryptophan as an excellent local molecular reporter for studies of a series of ultrafast protein dynamics, which include intraprotein electron transfer [64-68] and energy transfer [61, 69], as well as protein hydration dynamics [70-74]. As an optical probe, all these ultrafast measurements require no potential quenching of excited-state tryptophan by neighboring protein residues or peptide bonds on the picosecond time scale. However, it is known that tryptophan fluorescence is readily quenched by various amino acid residues [75] and peptide bonds [76-78]. Intraprotein electron transfer from excited indole moiety to nearby electrophilic residue(s) was proposed to be the quenching... 

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